shrink_fatten on GPencilStrokePoints
Your have a context issue because you might be running your script from text editor while shrink_fatten.poll method checks if you're in the 3D view.
Overriding context can be painful especially in edit mode, so you should avoid using ops whenever you can.
About the shrink_fatten ops, i noticed that it simply change the pressure value of each point. So i would try :
#set frame as the current active frame
frame = bpy.context.object.data.layers.active.active_frame

#set amount of pressure to add or remove
amount = 0.3 

#apply this to selected points
for stroke in frame.strokes :
    for point in stroke.points :
        if point.select :
            point.pressure += amount